An extrusion such as extruded polyvinyl chloride (PVC) pipe is formed by feeding plastic into an extruder where it is subjected to high temperatures to create a molten substrate. The substrate then proceeds through a feed tube, at the end of which additional material known as capstock can be applied if desired. The process of adding additional material to the substrate is known as co-extrusion. The substrate and capstock are known as melt. The melt proceeds through an extrusion head, at the end of which the melt passes through a die. The die contains the circular cross-sectional profile shape to be extruded. The melt hardens as it exits the die in the desired cross-sectional form. The hardened material forms a tube that can grow to arbitrary length as additional melt is extruded.
Normally, PVC pipe is produced by extruding molten plastic through a single die of an extruder. In some cases, two PVC pipes are produced simultaneously by extruding molten plastic through a Y-block, a pair of extrusion heads and a pair of dies.
The ultimate shape of the extrusion is determined by a melt flow passage in the die between a bushing which surrounds a pin or mandrel. For circular pipe, the pin is circular in cross-section and an opening in the bushing which surrounds the pin is circular. To obtain a non-circular cross-section, such as a square, a circular extrusion is typically reshaped through a transition bushing and pin which are circular at the inlet and square, for example, at the outlet.